Published: 2008-08-23 13:03:53
Updated: 2008-08-23 13:03:53
Posted August 23, 2008
By Paul Carter
MIKE MOSS SAYS: Paul, First I would note that we do include the Relative Humidity on our current conditions image, along with temperature, dew point, wind direction and speed and the current baraometric pressure and its tendency. Relative humidity can be an important variable to track when it comes to condensation or forming precipitation, and can be important to some people regarding health issues.
On the other hand, relative humidity can be a little confusing in the sense that it doesn't directly tell you anything about how much moisture is in the air. If I say the RH is 65%, you would have no way of knowing whether it's a sticky, humid day or if it is in fact very dry out, unless I also provide a temperature to go along with it.
That's fine, actually - we aren't "against" RH and "for" dew point, but dew point has an advantage for us in many cases because it changes only when the actual amount of water vapor in the air changes and it is directly related to the amount of water vapor, so that higher dew point = more water and vice versa, no matter what. However, higher RH = more water vapor (if temperature doesn't change), higher RH = less water vapor (if temperature decreases sufficiently), lower RH = less water vapor (if temperature doesn't change) and lower RH = more water vapor (if temperature increases sufficiently). Also, I left out that higher or lower RH can = no change in water vapor (if temperature change balances water vapor change). For this reason, suppose you graphed dew point and relative humidity for a typical week. You could look at the graph and know that increasing dew point means the airmass is changing toward a moister one, for example. The RH one on the other hand would go up and down sharply each day as the temperature rises and falls in response to sunrise and sunset, and if there was a gradual increase or decrease you wouldn't know if it was because of increasing moisture or decreasing temperatures.
In terms of comfort on a warm day, it turns out that regardless of the temperature once it's above about 80 degrees, most people feel "pleasantly dry" if the dew point is in the 50s or below, a little humid if dew point is in the low to mid 60s, noticeably humid if it is in the upper 60s to low 70s, and "steamy/oppressive" when it reaches the mid 70s and above.
I find this easier to relate to than remembering what RH percentage produces the same feeling for a given temperature. An illustration of this is to take a noticeably humid dew point of 70 degrees, say, on a day that reaches 90 degrees. The RH in that case would be 52% and the heat index 96. Suppose the temperature rose to 100 degrees. It would still feel quite humid to most people with the 70 degree dew point, and the heat index would now be 108 degrees, but the RH would now be 38%, even thought the amount of moisture in the air did not change.
One last item of interest on the subject of heat index. I find it handy to know that no matter what the temperature, if the dew point is above the upper 50s, the heat index will be greater than the actual temperature, while any lower value of dew point will fail to add any to how hot it "feels." So, 90 degrees and a dew point of 57 feels like 90 degrees. 100 degrees and a dew point of 58 also feels like 100 degrees. Using RH instead, at 90 degrees at an RH of 37% feels like 90 degrees, but to make 100 degrees feel like 100 degrees requires an RH of 25% while an RH of 37% would make for a 107 degree heat index.
I hope that gives you some idea of what we like about dew point and why we find it as valuable and in some cases more so in terms of ease of use than RH.